Electrode slurry coating apparatus and method for forming double active material layers

ABSTRACT

The present invention relates to electrode slurry coating apparatus and method, the present invention ultimately allowing the process efficiency to be increased and rate of errors to be reduced when double-layer structured active material layers are formed by temporally adjusting the height of first and second discharge outlets through which active material is discharged.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/KR2020/012451, filed on Sep. 16,2020, which claims priority to Korean Patent Application No.10-2019-0128891, filed on Oct. 17, 2019, the disclosures of which areherein incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an electrode slurry coating apparatusand method for forming an active material layer having a double layerstructure.

BACKGROUND ART

With the increase in technology development and demand for mobiledevices, the demand for secondary batteries is also rapidly increasing.Among them, lithium secondary batteries are widely used as an energysource for various electronic products as well as various mobile devicesbecause of their high energy density and high operating voltage andexcellent storage and lifetime characteristics.

In addition, the secondary battery has attracted attention as an energysource of an electric vehicle, a hybrid electric vehicle, etc., whichare proposed as a solution for air pollution of existing gasolinevehicles and diesel vehicles using fossil fuel. In order to be appliedas an energy source of an electric vehicle, a high-power battery isrequired.

In order to improve the performance of a secondary battery, thedevelopment of an electrode structure in which active material layershaving a two-layer structure are formed on a current collector isdrawing attention. A method of forming the two-layered active materiallayers on the current collector is to sequentially coat the slurryforming the lower and upper active material layers on the currentcollector in the form of a thin metal film. However, when the slurryforming the lower and upper active material layers is discharged at atime, the lower and upper active materials are mixed with each other, sothat a desired double-layer structure cannot be formed.

Therefore, when manufacturing an electrode having a double layerstructure of an active material, there is a need to develop a technologycapable of effectively forming an active material layer having a doublelayer structure in the electrode slurry coating process.

DISCLOSURE Technical Problem

The present invention was invented to solve the above problems, and anobject of the present invention is to provide an electrode slurrycoating apparatus and method for forming an active material layer havinga double layer structure with improved process efficiency.

Technical Solution

An electrode slurry coating apparatus 130 composed of a lower plate 131,a middle plate 132, and an upper plate 133 according to the presentinvention includes: a first discharge port 110 formed between the lowerplate 131 and the middle plate 132 and for discharging the slurryforming a lower slurry layer onto a current collector; a seconddischarge port 120 formed between the middle plate 132 and the upperplate 133, positioned to be spaced apart from the first discharge portin the downstream direction in the coating direction, and fordischarging the slurry forming an upper slurry layer onto the lowerslurry layer on the current collector; and a movement controller formoving the coating apparatus in a direction opposite to the dischargedirection.

In the present invention, the ends of the lower plate, the middle plate,and the upper plate are located on the same straight line.

In one embodiment, the movement controller controls the shortestdistance H1 between the end of the coating apparatus and the currentcollector to satisfy the following condition:

[Condition]

When a certain time elapses after forming the lower slurry layer, theapparatus is moved in an opposite direction to the discharge directionto form an upper slurry layer on the lower slurry layer, and at thistime, its moved distance H_(1T) is in a range of 60 to 140% of anaverage thickness of the upper slurry layer.

If the above range is less than 60%, compared to the amount of liquiddischarged, the space where the slurry stays to be coated, that is, thetotal area between the end of the coating apparatus and the lower slurrylayer, is insufficient, so that the supplied slurry cannot be coated andleaks back. On the other hand, in the case of more than 140%, comparedto the supplied slurry, the coating area is too large, so that thecoating may not be evenly performed, or only the lower layer may becoated and the upper layer may not be coated.

In another example, the time point at which the coating apparatus movesin a direction opposite to the discharge direction can be calculated bythe following Formula 1.movement switching point(T _(dS),sec)=(thickness(a) of middle plate(mm)+thickness (b) of first discharge port (mm))/moving speed (mm/sec)of current collector in the moving direction(MD).  [Formula 1]

In a specific example, a shortest distance (H_(1S)) between an end ofthe apparatus and the current collector before starting electrode slurrycoating is in a range of 60 to 140% of an average thickness of the lowerslurry layer.

If the above range is less than 60%, compared to the amount of liquiddischarged, the space where the slurry stays to be coated, that is, thetotal area between the end of the coating apparatus and the currentcollector, is insufficient, so that the supplied slurry cannot be coatedand leaks back. On the other hand, when it exceeds 140%, compared to thesupplied slurry, the coating area becomes too large, resulting in aphenomenon in which coating is not evenly performed.

In addition, the average thickness of each of the upper and lower slurrylayers is 40 to 200 PM.

Typically, the average particle diameter of the secondary battery activematerial is around 10 μm, but since the particle diameter follows anormal distribution, d(90) or d(max) is generally greater than 10 μm. Inorder to achieve good coating, in the present invention, the coatingapparatus is moved in the opposite direction to the discharge directionto form the upper slurry layer. At this time, when the average thicknessof the upper slurry layer is less than 40 μM, the moving distance H_(1T)is a value between 24 μM and 56 μM. In this case, as the moving distanceH_(1T) becomes closer to d (max), when the upper active material iscoated, the moving distance H_(1T) with the active material having themaximum particle diameter becomes close, and when the active materialcontained in the slurry is coated, there may be a phenomenon in which itis not possible to pass a height that is as high as H_(1T). It isbecause this may cause defects in the coating surface, for example, asituation that a line is formed on the coating surface because a largeactive material is caught, and it is caught between the moving currentcollector and the coating end and damages the current collector, therebycausing a rupture phenomenon of the current collector.

In addition, if the thickness of the slurry layer is 200 μm or more, itmay be advantageous, but there is a problem that it is difficult toachieve realistically to exceed 200 μm coating amount actually used forthe secondary battery.

In addition, the present invention a further includes: a first valveconfigured to open and close a discharge of the first discharge port; asecond valve configured to open and close a discharge of the seconddischarge port; and a valve controller configured to control opening andclosing of the first and second valves.

In a specific example, when the electrode slurry coating ends, the valvecontroller sets the closing time of the second valve to be delayed by aclosing delay time according to Formula 2 below from the closing time ofthe first valve.Upper slurry discharge closing delay time(T _(dT),sec)=(thickness(a) ofmiddle plate (mm)+thickness(b) of first discharge port (mm))/movingspeed (mm/sec) of current collector in the movingdirection(MD).  [Formula 2]

For example, the electrode slurry coating apparatus is a positiveelectrode slurry coating apparatus for a secondary battery.

The present invention also provides an electrode slurry coating methodusing the electrode slurry coating apparatus described above.

A method of coating an electrode slurry according to the presentinvention includes: forming a lower slurry layer by discharging a slurrythrough a first discharge port on a current collector moving in acoating direction (MD) by using an apparatus 130 for coating anelectrode slurry, composed of a lower plate 131, a middle plate 132 anda upper plate 133; moving the apparatus in a direction opposite to thedischarge direction; and forming an upper slurry layer by discharging aslurry through a first discharge port and a second discharge portpositioned to be spaced from a downstream side in a coating direction,on a lower slurry layer, wherein ends of the lower plate, the middleplate, and the upper plate are located on a same straight line.

In one example, the time point at which the coating apparatus moves in adirection opposite to the discharge direction can be calculated by thefollowing Formula 1.movement switching point(T _(dS),sec)=(thickness(a) of middle plate(mm)+thickness (b) of first discharge port (mm))/moving speed (mm/sec)of current collector in the moving direction(MD).  [Formula 1]

In another specific example, the electrode slurry coating methodaccording to the present invention starts discharging the slurry throughthe first discharge port when coating the electrode slurry, and when thecoating apparatus moves in a direction opposite to the dischargedirection, slurry discharge is started through the second dischargeport.

For example, a shortest distance (H_(1S)) between an end of theapparatus and the current collector before starting electrode slurrycoating is in a range of 60 to 140% of an average thickness of the lowerslurry layer.

If the above range is less than 60%, compared to the amount of liquiddischarged, the space where the slurry stays to be coated, that is, thetotal area between the end of the coating apparatus and the currentcollector, is insufficient, so that the supplied slurry cannot be coatedand leaks back. On the other hand, when it exceeds 140%, compared to thesupplied slurry, the coating area becomes too large, resulting in aphenomenon in which coating is not evenly performed.

In another example, the electrode slurry coating method according to thepresent invention is characterized in that, at the end of the electrodeslurry coating, the discharge stop time of the slurry forming the upperslurry layer is delayed by the valve closing delay time according toFormula 2 below than the discharge stop time of the slurry forming thelower slurry layer.Upper slurry discharge closing delay time(T _(dT),sec)=(thickness(a) ofmiddle plate (mm)+thickness(b) of first discharge port (mm))/movingspeed (mm/sec) of current collector in the movingdirection(MD).  [Formula 2]

In addition, the distance difference between the coating end point ofthe upper slurry layer and the coating end point of the lower slurrylayer according to the discharge delay of the present invention ispreferably within 3 mm.

In addition, the ratio of the average thickness (D₁) of the lower slurrylayer and the average thickness (D₂) of the upper slurry layer of thepresent invention is 1:3 to 3:1.

The thickness of the slurry layer as described above can be seen as thepressure of the immediately supplied slurry. In the case that thepressure of the upper slurry layer is supplied in excess of 3 times thepressure of the lower slurry layer so that the thickness ratio of thelower slurry layer and the upper slurry layer is 1:3 or more, since theupper layer has stronger pressure than the lower layer, the lower layerslurry is pushed back in the opposite direction to the coatingdirection, thereby increasing the possibility of leakage, and the lowerslurry may not be supplied properly due to the strong pressure of theupper slurry. In addition, due to the high pressure of the upper slurry,the supply of the slurry to the lower slurry layer is not uniform, sothat it is difficult to form the lower slurry layer uniformly.

On the other hand, when the pressure of the lower slurry layer issupplied in excess of 3 times the pressure of the upper slurry layer sothat the thickness ratio of the lower slurry layer and the upper slurrylayer is 3:1 or more, there is a problem that the supply of the upperslurry layer may become difficult, or the coating of the upper slurrylayer may be pushed in the coating direction, and the surface of thecoating liquid may be uneven.

Advantageous Effects

The electrode slurry coating apparatus and method according to thepresent invention can increase process efficiency and reduce defectrates when forming an active material layer having a double-layerstructure on a current collector. In addition, it is possible to reducethe electrode area discarded after the process by reducing the loadingoff section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an active material slurry coatingprocess using an electrode slurry coating apparatus according to anembodiment of the present invention.

FIG. 2 is a schematic diagram showing an electrode slurry coating methodaccording to an embodiment of the present invention.

FIG. 3 is a schematic view showing a cross section of an electrodemanufactured according to the electrode slurry coating method accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail withreference to the drawings. The terms and words used in the presentspecification and claims should not be construed as limited to ordinaryor dictionary terms and the inventor may properly define the concept ofthe terms in order to best describe its invention. The terms and wordsshould be construed as meaning and concept consistent with the technicalidea of the present invention.

In the present invention, “coating end” means not only the case ofterminating the electrode slurry coating, but also the case oftemporarily stopping the slurry coating. Specifically, it includes thecase of terminating or temporarily stopping the operation of theelectrode slurry coating apparatus, for example, the case of repeatingthe progress and interruption of slurry coating to form a patternedactive material layer, and the case of stopping the slurry coating.

In the present invention, “coating start” is meant to encompass not onlythe case of starting electrode slurry coating, but also the case ofresuming the temporarily stopped slurry coating. Specifically, itincludes the case of starting the operation of the electrode slurrycoating apparatus or restarting the operation that has been temporarilystopped, for example, the case of repeating the progress andinterruption of slurry coating to form a patterned active materiallayer, and the case of performing the slurry coating.

In addition, in the present invention, the “correspondence” of twospecific points is meant to encompass cases where the two points arelocated on the same line or within a similar range thereof. The factthat the two points are located on the same line includes not only thecase that they are physically located on the same line, but also a casethat they exist within an error range of facility or measurementequipment or a range including a buffer area of a certain level.

In general, in manufacturing an electrode, slurry including an activematerial, a conductive material, and a binder is prepared, dischargedonto a current collector to form a slurry layer, and finally, an activematerial layer (electrode layer) is formed through a drying process.

The present invention relates to an electrode slurry coating apparatusand a coating method for manufacturing an electrode having adouble-layer active material layer.

First, the present invention relates to an electrode slurry coatingapparatus 130 composed of a lower plate 131, a middle plate 132, and anupper plate 133, including: a first discharge port 110 formed betweenthe lower plate 131 and the middle plate 132 and for discharging theslurry forming a lower slurry layer onto a current collector; a seconddischarge port 120 formed between the middle plate 132 and the upperplate 133, positioned to be spaced apart from the first discharge portin the downstream direction in the coating direction, and fordischarging the slurry forming an upper slurry layer onto the lowerslurry layer on the current collector; and a movement controller formoving the coating apparatus in a direction opposite to the dischargedirection.

Characteristically, in the present invention, the ends of the lowerplate, the middle plate, and the upper plate are located on the samestraight line.

In one embodiment, the movement controller moves the electrode slurrycoating apparatus so that the shortest distance H1 between the end ofthe coating apparatus and the current collector satisfies the followingcondition.

[Condition]

When a certain time elapses after forming the lower slurry layer, thecoating apparatus is moved in the opposite direction to the dischargedirection to form the upper slurry layer on the lower slurry layer. Atthis time, the moved distance H_(1T) is in the range of 60 to 140% ofthe average thickness of the upper slurry layer. Preferably, the H_(1T)is in the range of 60 to 120% or 60 to 100% of the average thickness ofthe upper slurry layer. The H_(1T) provides a space in which the upperslurry layer is formed.

If the above range is less than 60%, compared to the amount of liquiddischarged, the space where the slurry stays to be coated, that is, thetotal area between the end of the coating apparatus and the lower slurrylayer, is insufficient, so that the supplied slurry cannot be coated andleaks back. On the other hand, in the case of more than 140%, comparedto the supplied slurry, the coating area is too large, so that thecoating may not be evenly coated, or only the lower layer may be coatedand the upper layer may not be coated.

Accordingly, in the electrode slurry coating apparatus according to thepresent invention, the lower and upper slurry layers are not mixed, andthe lower and upper slurry layers are then dried to stably form atwo-layer structure composed of lower and upper active material layers.

In the present invention, the average thickness of the upper slurrylayer is preferably 40 to 200 μm, more preferably 50 to 180 μm, and theaverage thickness of the lower slurry layer is 40 to 200 μm, morepreferably 50 to 180 μm.

Typically, the average particle diameter of the secondary battery activematerial is around 10 μm, but since the particle diameter follows anormal distribution, d(90) or d(max) is generally greater than 10 μm. Inorder to achieve good coating, in the present invention, the coatingapparatus is moved in the opposite direction to the discharge directionto form the upper slurry layer. At this time, when the average thicknessof the upper slurry layer is less than 40 μm, the moving distance H_(1T)is a value between 24 μm and 56 μm. In this case, as the moving distanceH_(1T) becomes closer to d (max), when the upper active material iscoated, the moving distance H_(1T) with the active material having themaximum particle diameter becomes close, and when the active materialcontained in the slurry is coated, there may be a phenomenon in which itis not possible to pass a height that is as high as H_(1T). It isbecause this may cause defects in the coating surface, for example, asituation that a line is formed on the coating surface because a largeactive material is caught, and it is caught between the moving currentcollector and the coating end and damages the current collector, therebycausing a rupture phenomenon of the current collector.

In addition, if the thickness of the slurry layer is 200 μm or more, itmay be advantageous, but there is a problem that it is difficult toachieve realistically to exceed 200 μm coating amount actually used forthe secondary battery.

In addition, the time point at which the coating apparatus moves in thedirection opposite to the discharge direction can be calculated by, forexample, Formula 1 below.Movement switching point(T _(dS),sec)=(thickness(a) of middle plate(mm)+thickness (b) of first discharge port (mm))/moving speed (mm/sec)of current collector in the moving direction(MD).  [Formula 1]

In the electrode slurry coating apparatus according to the presentinvention, the height H₁, which is the shortest distance between the endof the coating apparatus and the current collector, is changed fromH_(1S) to H_(1T) after the position (movement) change time calculated byFormula 1 above. Here, the shortest distance between the end of thecoating apparatus and the current collector means the length from thestraightened ends of the upper, middle and lower plates of the coatingapparatus to a vertical tangent to the current collector.

This is to stably form an upper slurry layer on the lower slurry layerformed after the lower slurry layer is formed first.

In one example, the shortest distance (H_(1S)) between the currentcollector and the end of the coating apparatus before starting electrodeslurry coating is controlled in the range of 60 to 140% of the averagethickness of the lower slurry layer, preferably 80 to 120%, and morepreferably 80 to 100%.

If the above range is less than 60%, compared to the amount of liquiddischarged, the space where the slurry stays to be coated, that is, thetotal area between the end of the coating apparatus and the currentcollector, is insufficient, so that the supplied slurry cannot be coatedand leaks back. On the other hand, when it exceeds 140%, compared to thesupplied slurry, the coating area becomes too large, resulting in aphenomenon in which coating is not evenly performed.

Meanwhile, the slurry is discharged from the first discharge port toform a lower slurry layer, and the slurry is again discharged from thesecond discharge port on the formed lower slurry layer to form an upperslurry layer. In the present invention, the slurry discharged from thesecond discharge port is designed to pressurize the lower slurry layerto a certain level. Through this, interlayer interfacial bondingproperties are improved, and air bubbles or the like are prevented frombeing formed at the interface.

In the present invention, the lower plate, the middle plate, and theends of the upper plate are characterized in that they are located onthe same straight line. In the present invention, the lower slurry layeris first formed through the first discharge port, and then the coatingapparatus is moved in the opposite direction to the discharge directionafter a certain time has elapsed, so that a space in which the upperslurry layer is formed is secured through the second discharge port.

In another example, an electrode slurry coating apparatus according tothe present invention further includes: a first valve for opening andclosing the discharge of the first discharge port; a second valve foropening and closing the discharge of the second discharge port; and avalve controller for controlling opening and closing of the first andsecond valves.

In addition, the valve controller opens the first valve when theelectrode slurry coating starts, and opens the second valve when thecoating apparatus moves in a direction opposite to the dischargedirection. This is to first form a lower slurry layer by opening thefirst valve. When the formed lower slurry layer reaches the position ofthe second discharge port by the movement of the conveyor that moves thecurrent collector, the second valve is opened at that time to stablyform the upper slurry layer on the lower slurry layer. In addition, bycontrolling the opening timing of the first and second valves, thecoating start points of the lower and upper slurry layers correspond toeach other, and the area of the surplus portion discarded through thismay be minimized.

In another example, for example, when the electrode slurry coating ends,the valve controller sets the closing time of the second valve to bedelayed by a closing delay time according to Formula 2 below from theclosing time of the first valve.Upper slurry discharge closing delay time (sec)=(thickness(a) of middleplate (mm)+thickness(b) of first discharge port (mm))/moving speed(mm/sec) of current collector in the moving direction(MD).  [Formula 2]

The closing delay time according to Formula 2 above is to minimize thearea of the surplus portion at the end time point of coating within arange that does not impede the stable formation of the two-layeredactive material layer.

Through the valve closing delay as described above, the distancedifference between the surplus portion, that is, the coating end pointof the upper slurry layer and the coating end point of the lower slurrylayer, is adjusted to be within 3 mm. This is because if the length ofthe surplus portion exceeds 3 mm as described above, the area to bediscarded increases, which is not economical.

In another embodiment, the ratio of the average thickness (D₁) of thelower slurry layer formed by the slurry discharged through the firstdischarge port and the average thickness (D₂) of the upper slurry layerformed by the slurry discharged through the second discharge port is inthe range of 1:3 to 3:1 (D₁:D₂). The thickness ratio is a relativeaverage value of the length of each layer in the thickness direction.

The thickness of the slurry layer as described above can be seen as thepressure of the immediately supplied slurry. In the case that thepressure of the upper slurry layer is supplied in excess of 3 times thepressure of the lower slurry layer so that the thickness ratio of thelower slurry layer and the upper slurry layer is 1:3 or more, since theupper layer has stronger pressure than the lower layer, the lower layerslurry is pushed back in the opposite direction to the coatingdirection, thereby increasing the possibility of leakage, and the lowerslurry may not be supplied properly due to the strong pressure of theupper slurry. In addition, due to the high pressure of the upper slurry,the supply of the slurry to the lower slurry layer is not uniform, sothat it is difficult to form the lower slurry layer uniformly.

On the other hand, when the pressure of the lower slurry layer issupplied in excess of 3 times the pressure of the upper slurry layer sothat the thickness ratio of the lower slurry layer and the upper slurrylayer is 3:1 or more, there is a problem that the supply of the upperslurry layer may become difficult, or the coating of the upper slurrylayer may be pushed in the coating direction, and the surface of thecoating liquid may be uneven.

The present invention also provides an electrode slurry coating methodusing the apparatus described above. In the detailed descriptionmentioned in the description of the apparatus or the specific numericalrange limitation, the overlapping portion will be omitted in thedescription of the electrode slurry coating method below.

The method of coating an electrode slurry according to the presentinvention includes: forming a lower slurry layer by discharging a slurrythrough a first discharge port on a current collector moving in acoating direction (MD) by using an apparatus 130 for coating anelectrode slurry, composed of a lower plate 131, a middle plate 132 anda upper plate 133; moving the apparatus in a direction opposite to thedischarge direction; and forming an upper slurry layer by discharging aslurry through a first discharge port and a second discharge portpositioned to be spaced from a downstream side in a coating direction,on a lower slurry layer.

In the present invention, the ends of the lower plate, the middle plate,and the upper plate are located on the same straight line.

In one example, in the electrode slurry coating method, the height (H1),which is the shortest distance between the end of the coating apparatusand the current collector, is changed from H_(1S) to H_(1T) at a pointin time after a certain time elapses after the start of electrode slurrycoating, and the H_(1T) provides a space in which an upper slurry layeris formed. Accordingly, in the electrode slurry coating method accordingto the present invention, the lower and upper slurry layers are notmixed, and a two-layer structure composed of the lower and upper slurrylayers is stably formed.

For example, the time point at which the coating apparatus moves in adirection opposite to the discharge direction can be calculated by thefollowing Formula.Movement switching point(T _(dS),sec)=(thickness(a) of middle plate(mm)+thickness (b) of first discharge port (mm))/moving speed (mm/sec)of current collector in the moving direction(MD).  [Formula 1]

In the electrode slurry coating method according to the presentinvention, the height (H₁) is changed from H_(1S) to H_(1T) after themovement conversion point calculated by Formula 1 above. This is tostably form an upper slurry layer on the lower slurry layer formed afterthe lower slurry layer is formed first.

In one example, the electrode slurry coating method according to thepresent invention starts discharging the slurry through the firstdischarge port when coating the electrode slurry, and when the coatingapparatus moves in a direction opposite to the discharge direction,slurry discharge is started through the second discharge port.

Through this, when the formed lower slurry layer reaches the position ofthe second discharge port by the movement of the conveyor that moves thecurrent collector, the second valve is opened at that time to stablyform the upper slurry layer on the lower slurry layer. In addition, bycontrolling the opening timing of the first and second valves like theabove, the coating start points of the lower and upper slurry layerscorrespond to each other, and the area of the surplus portion discardedthrough this may be minimized.

In another example, in one example, the shortest distance (H_(1S))between the current collector and the end of the coating apparatusbefore starting electrode slurry coating is controlled in the range of60 to 140% of the average thickness of the lower slurry layer,preferably 80 to 120%, and more preferably 80 to 100%. If the aboverange is less than 60%, compared to the amount of liquid discharged, thespace where the slurry stays to be coated, that is, the total areabetween the end of the coating apparatus and the current collector, isinsufficient, so that the supplied slurry cannot be coated and leaksback. On the other hand, in the case of more than 140%, compared to thesupplied slurry, the coating area is too large, so that the coating maynot be evenly performed, or only the lower layer may be coated and theupper layer may not be coated.

Here, the shortest distance between the end of the coating apparatus andthe current collector means the length from the straightened ends of theupper, middle and lower plates of the coating apparatus to a verticaltangent to the current collector.

The slurry is discharged from the first discharge port to form a lowerslurry layer, and the slurry is again discharged from the seconddischarge port on the formed lower slurry layer to form an upper slurrylayer. In the present invention, the slurry discharged from the seconddischarge port is designed to pressurize the lower slurry layer to acertain level. Through this, interlayer interfacial bonding propertiesare improved, and air bubbles or the like are prevented from beingformed at the interface.

In another example, the electrode slurry coating method according to thepresent invention is characterized in that, at the end of the electrodeslurry coating, the discharge stop time of the slurry forming the upperslurry layer is delayed by the valve closing delay time according toFormula 2 below than the discharge stop time of the slurry forming thelower slurry layer.Upper slurry discharge closing delay time (sec)=(thickness(a) of middleplate (mm)+thickness(b) of first discharge port (mm))/moving speed(mm/sec) of current collector in the moving direction(MD).  [Formula 2]

The closing delay time according to Formula 2 above is to minimize thearea of the surplus portion at the end time point of coating within arange that does not impede the stable formation of the two-layeredactive material layer.

The surplus portion as described above is referred to as the loading offsection, which means a section from the point at which the thickness ofthe slurry layer is reduced by stopping the discharge of the slurry, tothe end of the discharged slurry.

Through this, it is possible to show an effect of reducing the surplusportion that is discarded as the loading off section is caused. Thisleads to an increase in process efficiency and a decrease inmanufacturing cost. The loading off section means a section from thepoint at which the thickness of the slurry layer is reduced by stoppingthe discharge of the slurry, to the most end (end portion) of thedischarged slurry. In general, when the discharge closing is not delayedor is delayed too much, it is common that a loading off section of 5.5mm or more occurs.

Through the slurry discharge delay in the upper slurry layer asdescribed above, the distance difference between the surplus portion,that is, the coating end point of the upper slurry layer and the coatingend point of the lower slurry layer, is adjusted to be within 3 mm. Thisis because if the length of the surplus portion exceeds 3 mm asdescribed above, the area to be discarded increases, which is noteconomical.

FIG. 3 shows a case in which the lower slurry layer 111 and the upperslurry layer 121 are sequentially coated on a current collector movingin the coating direction MD by a conveyor and are terminated. Bydelaying the discharge of the slurry from the upper slurry layer asdescribed above, the distance difference between the coating end point(E bottom) of the lower slurry layer 111 and the coating end point (Etop) of the upper slurry layer 121 can be reduced.

In addition, it is possible to reduce the length of the surplus portioncompared to the prior art by delaying the discharge of the slurry in theupper slurry layer as described above. Here, the loading off sectionmeans the total distance from the portion where the thickness of theslurry layer starts to decrease (E terminal) to the end (E top) of theslurry.

In the present invention, while the E bottom and E top coincide, thelength of the loading off section is reduced at the same time.

In another embodiment, the ratio of the average thickness (D₁) of thelower slurry layer formed by the slurry discharged through the firstdischarge port and the average thickness (D₂) of the upper slurry layerformed by the slurry discharged through the second discharge port is inthe range of 1:3 to 3:1 (D₁:D₂). The thickness ratio is a relativeaverage value of the length of each layer in the thickness direction.

Hereinafter, the present invention will be described in more detailthrough drawings and examples.

FIGS. 1 and 2 are schematic diagrams showing an active material slurrycoating process using an electrode slurry coating apparatus according toan embodiment of the present invention. Referring to FIG. 1 , theelectrode slurry coating apparatus includes a lower plate 131 and anupper plate 133, and a middle plate 132 is interposed between the lowerplate 131 and the upper plate 133. A slurry including an activematerial, a conductive material, and a binder fluidly moves along a flowpath between the lower plate and the middle plate 131 and 132, and theslurry forming the lower slurry layer 111 is discharged through thefirst discharge port 110. A slurry including an active material, aconductive material, and a binder fluidly moves along the flow pathbetween the middle plate and the upper plates 132 and 133, and theslurry forming the upper slurry layer 121 is discharged through thesecond discharge port 120. In addition, a conveyor (not shown) formoving the current collector 101 in the coating direction MD is locatedspaced apart from the first and second discharge ports 110 and 120 by apredetermined distance.

At this time, the ends of the lower plate, the middle plate and theupper plate of the coating apparatus are located on the same straightline.

In addition, referring to FIG. 2 , in the coating apparatus, the ends ofthe apparatus, that is, the ends of the lower plate, the middle plate,and the upper plate are spaced apart from the current collector 101 by apredetermined distance. Herein, it is spaced apart before the start ofcoating by the shortest distance H_(1S) between the end of the coatingapparatus and the current collector.

The slurry discharged through the first discharge port 110 forms a lowerslurry layer 111 having an average thickness D₁ on the current collector101 and makes the coating apparatus to be separated from the currentcollector by a predetermined distance through a movement controller (notshown) that moves the coating apparatus in a direction opposite to thedischarge direction. Thereafter, the slurry discharged through thesecond discharge port 120 forms an upper slurry layer 121 having anaverage thickness D₂ on the lower slurry layer 111.

First Embodiment

A positive electrode for a lithium secondary battery was manufacturedthrough the electrode slurry coating apparatus and method shown in FIG.1 . Specifically, when starting electrode slurry coating, the shortestdistance H_(1S) between the surface of the current collector 101 movingalong the conveyor and the end of the coating apparatus is 80 μm. Theslurry is discharged from the first discharge port 110 to form a lowerslurry layer. Then, the height H₁ of the coating apparatus is moved inthe opposite direction to the discharge direction by H_(1S) to H_(1T) asshown in FIG. 2 . The time point at which the coating apparatus moves iscalculated by Formula 1 below. Specifically, the moving speed of thecurrent collector 101 by the conveyor was 50 m/min, the thickness (a) ofthe middle plate was 1 mm, and the thickness (b) of the first dischargeport was also 1 mm. Applying this to Formula 1 is as follows.Movement switching point(T _(dS),sec)=(thickness of middle plate(mm)+thickness of first discharge port (mm))/moving speed (mm/sec) ofcurrent collector in the moving direction (MD).  [Formula 1]

The sum of the thickness of the middle plate and the thickness of thefirst discharge port is 2 mm. In addition, moving speed (mm/sec) of thecurrent collector 101 in the moving direction (MD) by the conveyor is 50(m/min), that is, 83.3 (mm/sec). If calculated according to Formula 1,the travel time (T_(dT)) is 2.4×10⁻³ (sec), that is, 2.4 ms(milliseconds).

H_(1T), which is the distance the coating apparatus was moved in thedirection opposite to the discharge direction during the above time, was60 μm.

The overall average thickness (DT) of the slurry double layer coated bythe electrode slurry coating apparatus is about 150 μm. Among them, theaverage thickness of the lower slurry layer D1 is 90 μm, and the averagethickness of the upper slurry layer D2 is 60 μm.

Second Embodiment

A positive electrode for a lithium secondary battery was prepared usingthe electrode slurry coating apparatus shown in FIG. 1 . A detaileddescription of the electrode slurry coating method is omitted since itoverlaps with the first embodiment.

However, H_(1T), the distance by which the coating apparatus was movedin the opposite direction to discharge direction, which was the startingdistance, was 90 μm.

The overall average thickness (DT) of the slurry double layer coated bythe electrode slurry coating apparatus is about 180 μm. Among them, theaverage thickness of the lower slurry layer D1 is 90 μm, and the averagethickness of the upper slurry layer D2 is 90 μm.

Third Embodiment

A positive electrode for a lithium secondary battery was prepared usingthe electrode slurry coating apparatus shown in FIG. 1 . A detaileddescription of the electrode slurry coating method is omitted since itoverlaps with the first embodiment.

However, at the end of the electrode slurry coating, the closing time ofthe second valve was delayed by the closing delay time according toFormula 2 below from the closing time of the first valve. Specifically,the moving speed of the current collector 101 by the conveyor was 50m/min, the thickness (a) of the middle plate was 1 mm, and the thickness(b) of the first discharge port was also 1 mm. Applying this to Formula2 is as follows.Upper slurry discharge closing delay time (sec)=(thickness(a) of middleplate (mm)+thickness(b) of first discharge port (mm))/moving speed(mm/sec) of current collector in the moving direction(MD).  [Formula 2]

The sum of the thickness of the middle plate and the thickness of thefirst discharge port is 2 mm. In addition, moving speed (mm/sec) of thecurrent collector 101 in the moving direction (MD) by the conveyor is 50(m/min), that is, 83.3 (mm/sec). If calculated according to Formula 2,the valve closing delay time is 2.4×10⁻³ (sec) or 2.4 ms (milliseconds).

Therefore, at the end of the electrode slurry coating, the closing timeof the second valve was delayed by 2.4 ms from the closing time of thefirst valve. In this case, in the manufactured electrode, the coatingend points of the lower and upper slurry layers 111 and 121 wereidentical (E top=E bottom), and as a result of measuring the loading offlength, which is the distance from the portion (E terminal) where thethicknesses of the upper and lower slurry layers are reduced to the endportion (E top, E bottom) where the coating is finished, it was formedas 4.5 mm.

In general, the loading off length is formed to be 5.5 mm or more, andin the case of manufacturing according to the third embodiment, aportion to be discarded is saved by reducing the length of the surplusportion, thereby increasing the efficiency of the process.

In the above, the present invention has been described in more detailthrough the drawings and examples. Accordingly, the embodimentsdescribed in the specification and the configurations described in thedrawings are only the most preferred embodiments of the presentinvention, and do not represent all of the technical ideas of thepresent invention. It is to be understood that there may be variousequivalents and variations in place of them at the time of filing thepresent application.

DESCRIPTION OF REFERENCE NUMERALS

-   -   101: current collector    -   110: first discharge port    -   111: lower slurry layer    -   120: second discharge port    -   121: upper slurry layer    -   131: lower plate of coating apparatus    -   132: middle plate of coating apparatus    -   133: upper plate of coating apparatus    -   a: thickness of the middle plate    -   b: thickness of the first discharge port

The invention claimed is:
 1. A method of coating an electrode slurry,the method comprising: forming a lower slurry layer by discharging aslurry through a first discharge port on a current collector moving in acoating direction (MD) by using an apparatus for coating an electrodeslurry, composed of a lower plate, a middle plate and a upper plate;moving the apparatus in a direction opposite to a discharge direction,after starting to form the lower slurry layer and before forming anupper slurry layer; and forming the upper slurry layer by discharging aslurry through a second discharge port positioned to be spaceddownstream side in a coating direction from the first discharge port,wherein ends of the lower plate, the middle plate, and the upper plateare located on a same straight line.
 2. The method of claim 1, wherein atime point after starting to form the lower slurry layer at which theapparatus moves in the direction opposite to the discharge direction iscalculated by following Formula 1:movement switching point (T _(dS), sec)=(thickness (a) of the middleplate (mm)+thickness (b) of the first discharge port (mm))/moving speed(mm/sec) of the current collector in the moving direction(MD).  [Formula 1]
 3. The method of claim 1, wherein when an electrodeslurry coating is started, the slurry is discharged through the firstdischarge port, and wherein when the apparatus moves in a the directionopposite to the discharge direction, the slurry is discharged throughthe second discharge port.
 4. The method of claim 1, wherein a shortestdistance (His) between an end of the apparatus and the current collectorbefore starting electrode slurry coating is in a range of 60 to 140% ofan average thickness of the lower slurry layer.
 5. The method of claim1, wherein at an end of electrode slurry coating, time to stop adischarge of the slurry forming the upper slurry layer is delayed by avalve closing delay time according to Formula 2 below:upper slurry discharge valve closing delay time (sec)=(thickness (a) ofthe middle plate (mm)+thickness (b) of the first discharge port(mm))/moving speed (mm/sec) of the current collector in the movingdirection (MD).  [Formula 2]
 6. The method of claim 5, wherein adistance difference between a coating end point of the upper slurrylayer and a coating end point of the lower slurry layer according to thedischarge delay is within 3 mm.
 7. The method of claim 1, wherein aratio of an average thickness (D₁) of the lower slurry layer and anaverage thickness (D₂) of the upper slurry layer is 1:3 to 3:1.